Electrical condenser



Oct. 8, 1940. ROBINSON 2,217,383

ELECTRICAL CONDENSER Filed Aug. 1, 1939 IIIIIIIIIIIIIIIIIIIIIIIIA IIIIIIIIIIIIIIIIIIIIIIIII PRESTON ROBINSON 1N VENTOR.

(W 6 *g TTORNEYS Patented Oct.8,1940 2,217,383

UNITED STATES PATENT OFFICE 2,217,383 ELECTRICAL CONDENSER Preston Robinson, Williamstown, Mass., assignor to Sprague Specialties 00., North Adams, Mass., a corporation of Massachusetts Application August 1, 1939, Serial No. 287,829

12 Claims. (01. 175-41.5)

The present invention relates to electrical confect contact between the electrodes and the didensers, and more particularly to novel means electric due to the microscopically rough surfaces and processes for improving the stability of conof the electrodes and the air pockets which are densers. The invention will be described in conformed in the minute crevices of such rough surnection with condensers of the type commonly face. By expanding and contracting with 5 known as trimmer condensers, which find their changing temperature, the air so held changes chief application as semi-adjustable means for the contact area between the electrodes and the tuning the resonant circuits of radio receiver to dielectric and also the effective thickness of the fixed selected frequencies. dielectric. Because of this the condenser exhib- 0 Such trimmer condensers comprise as their its ahigh temperature coefiicient of capacity, and main elements two electrodes disposed adjustably furthermore because of the imperfect contact bewith respect to each other with a dielectric layer, tween the electrode and the dielectric, the full for example, a sheet of mica, interposed between capacity of the condenser cannot be realized.

same. Usually the electrodes and the interposed My invention obviates the above-recited drawdielectric form a stack which is secured to a suitbacks and insures a great degree of stability in 15 able base of low-loss insulating material. capacity. This I achieve by interposing between The chief drawback of present-day trimmer the electrodes and the dielectric an electrical concondensers is that they are diflicult to adjust to ducting material which is soft, non-thermoplasand do not retain their desired capacity setting. tic, has a low coefficient of friction, and main- Since the instability in capacity both with retains its desirable properties, wear and atmos- 20 gard to initial setting and temperature variapheric influences notwithstanding. tions is more pronounced the higher the capacity Such a material, of which there are a number setting of the condenser, it has been the practice but of which I have found a suitably deposited to operate trimmer condensers within a range graphite particularly advantageous, I dispose as a limited by the so-called working" capacity value. substantially unaltering and lasting coating or 25 The working capacity of a trimmer condenser, layer on the surface of the electrodes in a manner however, is much less than is its maximum obhereinafter to be more fully described. tainable capacity, and to obtain satisfactory sta- Such interposition of a soft electrical conductbility, in many cases, for example, in the coming material of great permanency and having a pression type of trimmer condenser, only 40% to low coefficient of friction, between the surfaces 50% of the inherent capacity range of a conof the electrodes and the dielectric, brings about denser can be utilized. the desired high degree of stability of a trimmer I have found that the instability exhibited by condenser because of the following: prior trimmer condensers is attributed to several First, the soft material so interposed between factors. One of these factors is the difference in the electrodes and the dielectric, serves as a 35 the physical hardness between the metal elecbuffer or cushion between these elements and trodes and the dielectric, for example, mica. prevents the sharp ragged surface of the elec- This causes the electrodes to cut or wear into the trodes from physically contacting'with and cutdielectric at their contacting portions particuting into the dielectric.

40 larly when there is a change in their relative po- Secondly, the soft conducting material fills 4Q sltion either due to adjustment or to expansion the minute crevices of the electrode surface, and contraction of these parts because of temeliminating air pockets and such large capacity perature changes. changes as are otherwise brought about by the A second factor which I have found to conexpansion and contraction of the air due to temtribute to the instability of condensers, is the perature changes. 45

high friction coefficient between the dielectric Thirdly, the useof a material having a low. co-

and the electrodes. ,Because it initially resists efilcient of friction permits unimpeded moveany relative movement of the parts, this friction ment of the electrode and dielectric relative to sets uplateral stresses therebetween, which subeach other and eliminates lateral stresses and sequently may be removed either wholly or in hysteresis in these elements which would be 50 part, ornot removedat all, bringing about an otherwise set up during adjustment of the caunpredictable change in the capacity of the conpacity of the condenser or during the expansion denser. and contraction of the surfaces caused by A third factor contributing to the instability changes in temperature. of prior trimmer. condensers is due to the imper- In addition to a high stability, several other 55 important advantages are achieved by means of my invention. For example, due to the intimate molecular contact which it provides throughout the contacting surfaces of the electrodes and the dielectric, my interposed soft conducting layers give higher maximum capacity values than have otherwise identical condensers lacking such layers.

Furthermore, because of the high degree of stability at even the maximum capacity setting, the full capacity range of the condenser can be actually utilized in practice.

As the material for the soft electrical conducting layer having the above requirements I have found graphite the most suitable. However, other conducting materials, for example, lead peroxide, magnetite, etc., can also be used.

To obtain the benefits of these materials and to maintain these desirable properties thereof, it is essential and so part of this invention, to so provide the layers comprising same that they retain their function and desirable properties unaltered during the life of the condenser.

I have found, for example, that if graphite is merely smeared or applied by one of the usual processes to the metal electrodes, it will gradually loosen from the coated surface and part of it will actually deposit on the dielectric, thereby defeating the purpose of the invention by decreasing rather than increasing the stability of the condenser.

Moreover, graphite layers so provided are affected by moisture and atmospheric condition. nor do they adhere to the edges and burrs of the electrode plates and therefore lack the protective features aimed at. To insure the permanency of the coating and its other desirable properties such as moisture resistance, the coating has to be applied to the electrode by a method which renders it well adhering to the electrode plate so as to form an integral coating thereon. A method which suggests itself is to mold the graphite or similar material with a suitable binder around the electrode. However, this method has the drawback that only comparatively thick coatings can be formed and these introduce an undesirable ohmic resistance in the condenser.

The process which I have found in every way satisfactory for applying such coatings and for obtaining permanency in their desired characteristics is by means of cataphoretic deposition.

My invention will be further described with reference to the appended drawing, in which:

Figure 1 is a cross-sectional view of a trimmer condenser embodying my invention.

Fig. 2 is a top plan view of the condenser of Fig. 1.

Fig. 3 is an enlarged cross-section of a portion of one of the metal electrodes of the condenser of Figs. 1 and 2, provided with a conducting layer or coating in accordance with the invention.

Referring now to Figs. 1 and 2, the condenser shown therein comprises a base I! of Bakelite," Isolantite, or other suitable low dielectric loss insulating material. Disposed on the base II is a fixed electrode plate ii and a resilient metal electrode plate i2. The plates II and I! are each provided with a surface coating of a soft non-thermoplastic, low-friction electrical conducting material it and are to be later more fully referred to.

Interposed between the plates II and i2 is a layer of a suitable dielectric material It, for example, a sheet of mica.

The design of the electrode pistes II and I! may conform with any accepted practice. As shown, the fixed electrode plate ii comprises two integral portions 20 and 21. The portion 20 is the effective portion of the electrode whereas the portion 21 serves as one terminal for the condenser.

The electrode Ii is fixedly held to the base I. by means of a rivet II or other attachment means which passes through the base II, the portion II and the dielectric H.

The adjustable electrode plate I! consists of s. substantially flat portion 20. two arched flexing members 2 l-2l, a supporting and mounting portion 22, and a terminal portion 23.

The portion 20 forms the effective portion of the electrode and its capacitative relationship to the electrode plate I l is varied by changing the effective distance between these plates; the arched members Ii-li imparting the necessary resiliency or "spring" to the portion 2..

Suitable electrical connection to the plate I! is made by means of the terminal 23 which consists of a rectangular member disposed in a plane substantially perpendicular to the portion II.

The plate I! is held to the base II by means of a rivet 24 which passes through the base II, the dielectric I4, and the mounting portion 22. The said mounting portion is integral with the terminal portion 23 and projects perpendicularly therefrom whereby its position in the assembly of the condenser is in the same plane as the flat portion 1..

The base II is provided at one end with a raised portion 25 of a height corresponding to the thickness of the fixed electrode ii whereby the dielectric member It presents a level surface to the bottom face of the electrode II.

For the heating of the plate I! there is provided a metal adjustment screw ll having a head it. Interposed between the screw head I. and the flexible plate I! is a washer I! which may be either of metal or of an insulating material. The screw ii threaded engages a nut ll provided in a recess I! of the base II.

The electrode I2 is provided centrally at its portion III with an aperture sumciently large to permit non-contacting passage of the adjustment screw II. A similar aperture is provided in the portion 26 of the electrode II.

The electrodes Ii and I! as previously described, as provided with an integral surface coating of a soft non-thermoplastic, low friction electrical conducting material, and preferably of graphite. A coating provided in accordance with my invention is shown in Fig. 3 in which a metal electrode shown in portion as II, has disposed about it a coating II which fills the crevices of its microscopically rough surface.

In Fig. 3, the rough character of the electrode surface and the thickness of the coating II has been exaggerated for the purposes of illustration.

The graphite coating II is provided preferably on both electrodes of the condenser; however, I have found that a high degree of stability in the condenser is achieved even if only the adjustable electrode i2 is provided on its surfaces with such a coating.

To achieve a condenser of high stability in accordance with the invention, the coating ll must be integral with and permanently adhering to all portions of the electrode surface. A process which I have found in every way satisfactory to obtain such a coating is by cataphoretic deposition such as described in detail in the copending appliestion of Preston Robinson and Stanley 0. Durst, s. N. 197,692, filed March 23, 1988.

To adapt the said process for the purpose of the present invention, I prepare a coating solution comprising one part of the commercial prodnot known as Aquadag or a similar suspension of graphite having imparted to it colloidal properties by the addition of a peptiaing or emulsifying agent such as tannic acid, in ten to twenty parts of distilled water. The-electrode to be coatedismadetheanodeinacoatingcellto which the suspension has been added. By the application of approximately 40 volts to the cell for approximately one second a uniform graphite coating of from .0001" to .0004" thick, is deposited on the surface of the electrode.

The voltage used and the time of deposition may, of course. vary from that above recibd. In general. the thickness of the coating deposited is roughly proportional to the voltage and the time of deposition whereby to obtain a coatind of a given thickness in a shorter time, a correspondingiy higher voltage must be used.

It should be noted that by the use of cataphoretic deposition the coating so obtained covers 'not only the flat surfaces but also covers to the same depth or more all edges, corners, and burrs of the electrode. I

The coating in general should have a thickness of at least the order of magnitude of the burrs or other microscopic roughnesses which exist on the electrode to be coated.

Coatings having a thickness greater than above recited may also be used, however, I have found that thick coatings may tend to peel and fiake from the surface of the electrode. Moreover. since the material comprisin the coating has generally a lower electrical conductivity than the underlying electrode metal the use of thick coat-v ings tends towards an increase in the power factor of the condenser.

The coating so deposited is dried to remove the small quantity of water contained on its surface and then baked to the underlying electrode metal in a suitable oven heated to a temperature of approximately 150 C.

The baking at 150 C. may take place for onehalf to one hour or more at the end of which time it is found that the graphite coating is of great permanency and does not brush off the surface of the coated electrode. Nor will the immersion of such a coating in boiling water for a period of fifteen mintues or even more cause it to loosen and part from the surface of the electrode.

The reasons for these remarkable properties in the coating of my invention I do not fully understand, however, I believe important contributing factors to be that the graphite particles are oriented partly at least in the same direction by the electric field during their cataphoretic deposition, and that the structure of the coating so obtained is preserved by the baking and by the removal or destruction of the emulsifying agent during the baking.

Trimmer condensers the. electrodes of which are provided in accordance with my invention, with a soft non-thermoplastic electrical conducting coating having a low coefiicient of friction, exhibit a high degree of stability when compared with condensers of similar mechanical construction lacking suchelectrode coatings.

ratio unpredictable capacity. changes of from 110% to 4% to a uniform predictable change of approximately 8% to 5%. The condensers so compared were subjected after an initial setting of their capacity value to two heat cycles of from 25 C. to 60 C.

Trimmer condensers the elements of which comprisedissimilarmaterialswillonheatingand cooling exhibit a temperature coemcient of capacity due to the change in dimensions of the elements. Condensers of the prior art even apart from the previously described factors bringing about their instability, exhibit different capacity values at a given temperature depending on whether the temperature is approached from above or below. This phenomenon is called hysteresis. Condensers the electrodes of which are provided with an integral coating in accordance with my invention are singularly free from hysteresis.

Moreover, whereas in prior trimmer condensersitisnecessarytomakeseveralreadjustments of the capacity adjustment means to obtain the desired capacity value, a condenser provided with the coating of the invention is free of this drawback and its desired capacity value can be established by a single adjustment of the adjustment means.

Furthermore, partly because of the intimate contact provided between the contacting surfaces of the electrode and the dielectric, and partly because of the unimpeded movement of these elements brought about by the coating. the same torque-a torque of one inch pound-applied to the adjustment means produces nearly more capacity.

Because of the high degree of stability achieved the working capacity range of the condenser can be extended to this capacity value.

It is thus evident that by means of my novel invention, I have been able not only to increase the stability of the condenser and its maximum capacity value but also its maximum utilizable capacity as well.

While I have described my invention by means of a specific example and in a specific embodiment, I do not wish to be limited thereto for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

Wh t I claim is:

1. In a condenser, two electrodes, and a dielectric layer interposed between same, said electrodes provided on the enga i surfaces thereof with an integral coating of a soft non-thermoplastic electrically conducting material having a low coeflicient of friction. w

2. In an adjustable condenser, two electrodes with means for adjusting their relative position and a dielectric layer interposed between said electrodes, means to increase the stability of said condenser, said means comprising a soft nonthermoplastic electrically conducting material having a low coeiiicient of friction integrally disposed on said electrodes and interposed between the engaging surfaces of said electrodes and said dielectric.

3. In a trimmer condenser an electrode provided on its active surfaces with an integral coating of graphite.

' 4. In a trimmer condenser a stationary electrode, an adjustable electrode and a dielectric layer interposed between same. said electrodes provided on the enga in surfaces thereof with an integral coating of graphite .0001" to .0004" thick.

5. In a trimmer condenser a fixed electrode, an adjustable electrode and a dielectric layer interposed between said electrodes, and means to prevent said electrodes from cutting into said dielectric, said means comprising a layer of an electrical conducting material which is softer than said dielectric integrally disposed on said electrodes and interposed between said electrodes and said dielectric.

6. In a trimmer condenser a fixed electrode, an adiustable electrode and a dielectric layer disposed between said electrodes, and means to permit unimpeded lateral movement of said electrodes with respect to said dielectric, said means comprising an electrical conducting material having a low coefilcient of friction integrally disposed on said electrodes and interposed between said electrodes and said dielectric.

7. In a trimmer condenser a fixed electrode, an adjustable electrode and a dielectric layer interposed between said electrodes, means to permit an intimate contact between said electrodes and said dielectric said means comprising a layer of an electrical conducting material which is softer than said dielectric integrally disposed on the engaging surfaces of said electrodes.

8. In a trimmer condenser, a fixed electrode,- an adjustable electrode and a dielectric layer interposed between same, an integral coating of graphite disposed on the surface of at least one of said electrodes and interposed between said electrode and said dielectric layer, said coating remaining substantially unefi'ected when sub- Jected to immersion in boiling distilled water.

9. In a trimmer condenser, a fixed electrode,

an adjustable electrode and a dielectric layer interposed between same, an integral coating disposed on the surface of one of said electrodes and interposed between said electrode and said dielectric layer, said coating comprising graphite the particles of which have been oriented on the surface of the electrode by the action of an electric field.

10. A hysteresis free adjustable condenser, comprising two electrodes with means for ad- Justing their relative position, a dielectric layer interposed between said electrodes and an integral coating of graphite disposed on one of said electrodes and interposed between said electrode and said dielectric layer.

11. In a trimmer condenser, a fixed electrode, an adjustable electrode and a dielectric layer interposed between same, an integral coating of graphite devoid of emulsifying agents disposed on the surface of at least one of said electrodes and interposed between said electrode and said dielectric layer.

12. In a trimmer condenser, a fixed electrode, an adjustable electrode and a dielectric layer interposed between same, said electrodes having surfaces with microscopic roughnesses, and an integral coating of graphite having a thickness of at least the height of said roughnesses provided on the engaging surfaces of said electrodes.

PRESTON ROBINSON. 

